6200
J . Am. Chem. SOC.1984, 106, 6200-6206
Ester 11 is an extreme, but not isolated, example of this effect, where the very long *C-0 bond length expected for the ester of a strong acid and a tertiary alcohol is not observed when strongly electron-withdrawing substituents (CF3, C=O, 0) are present at C* or on the adjacent carbon atom.20s21-z2 A similar effect no doubt accounts for the very short *C-OMe bond in the methyl ether (12, Table 11). We also describe in the following paper in this issue a detailed examination of a comparable series of acetals and glucosides, where the four substituent OH groups of the glucosides similarly inhibit the lengthening of the bond from the anomeric center to good leaving groups, OR2. We conclude that systems Rl-OR2 respond to increasing electron withdrawal in the group ORz by electronic reorganization in the direction of the valence bond tautomer (5), resulting in the observed increase in the length of the R l - O R z bond and charge separation in the sense Rl+-ORz. This must mean that a substantial part of the high reactivity toward heterolysis of tertiary alkyl compounds with good leaving groups derives from the substantial amount of bond breaking apparent in the ground state, compared with less reactive compounds. (Estimates of the variation of bond energy with bond length in systems of this sort are of the order of 2-300 kcal mol-’ A-’,3,7*23 while bond length differences across a series may easily amount to 0.06 A (Table I) or more.) Evidently estimates of the extent of bond breaking in the transition state, which are commonly made on the basis of comparisons of reactivity over a series of compounds, should take into account the varying amounts of “bond breaking” in the ground state. Protonation of oxygen results in an enormous increase in electron demand so that much-perhaps most-of the C-0 bond-breaking process in acid-catalyzed reactions of tertiary alcohol derivatives may be expected to take place as part of the preequilibrium proton-transfer step. In fact, in any comparison of transition states for a given reaction of two different compounds, (20) Smith, G. D.; Caughlan, C. N.; Ramirez, F.; Glaser, S. L.; Stern, P. J . A m . Chem. SOC.1974, 96, 2968. (21) Ramirez, F.; Ricci, J. S.; Madan, 0. P.; Marecek, J. F.; Tsuboi, H. J . A m . Chem. SOC.1977, 99, 5135. (22) Doesburg, H. M.; Petit, G. H.; Merckx, E. M. Acra Crystallogr., Sect 8 1982, 838, 1181. (23) Bartell, L. S. Tetrahedron 1962, 17, 177.
it may be of crucial importance to consider the status of the bond or bonds being broken in the ground state. We showed recently in our work on acetal hydrolysis that the secondary deuterium isotope effect for the spontaneous hydrolysis of axial p-nitrophenyl acetal (13a) is significantly smaller.than that of the equatorial isomer (13e).
13a
13e 1.20
1.29
It was tempting to attribute this difference to a stereoelectronic effect on transition states, though much other evidence7suggested that the transition states for the reactions of the two isomers were closely similar. But our X-ray structural work4 shows clearly that the C-OAr bond of the axial isomer is expected to be substantially the longer (1.448 (4) 8, for an analogue of 13a, Ar = p-nitrophenyl, compared with 1.424 (4) 8, for 13e). The original state of the axial isomer is thus closer in geometry to the transition state: compared with the equatorial compound it starts farther along the reaction coordinate for C-OAr cleavage, so that the change in the character of the C-H bond between the ground and transition state is reduced. The effect is indeed stereoelectronic in origin, but it is evidently an effect on ground states. Our results indicate, further, that it should be possible by accurate crystal structure determinations, using an appropriate series of compounds based on an oxygen probe with increasing electron demand, to explore the early stages of bond breaking in many organic reactions which are initiated by ionization to form a carbocation. (This is an extension of the approach pioneered by Burgi and Dunitz,lb who found that similar changes in the lengths of the Y-M bonds of inorganic systems, YMX,, are related to bond angle changes in the MX3 fragment.) In principle it should be possible to observe how the carbon fragment R1 accomodates the developing positive charge. In the following paper in this issue4 we describe a detailed examination of bond length and reactivity in acetals and glucosides using this approach. We are currently examining several other series, particularly systems which undergo rearrangement and fragmentation reactions.
Bond Length and Reactivity. Stereoelectronic Effects on Bonding in Acetals and Glucosides Andrew J. Briggs,*Robert Glenn,* Peter G. Jones,? Anthony J. Kirby,** and P. Ramaswamy* Contribution from the University Chemical Laboratory, Cambridge CB2 1 E W, England, and the Institut fur Anorganische Chemie, 0-3400 Gottingen, Federal Republic of Germany. Received October 12, 1983 Abstract: Accurate X-ray crystal structure determinations for 22 axial and equatorial tetrahydropyranyl acetals and a-and @-glucopyranosidesreveal systematic changes in the pattern of bond lengths at the acetal center with changing electron demand in the exocyclic (“leaving”)group. Stereoelectronic effects on bonding are analyzed and related to reactivity. Linear correlations between the pK, of the conjugate acid of the leaving group and hence the free energy of activation for cleavage of the acetal C-0 bond and the length of the bond being broken appear to be the rule rather than the exception over the range of leaving group studied.
Our recent on the dependence of reactivity on conformation in acetal hydrolysis has shown that the cleavage of acetals is subject to stereoelectronic c o n t r 0 1 . ~ ~C-0 ~ cleavage occurs readily only when a nonbonding electron pair (lone pair) on the remaining. oxygen atom of the 0-c-0 group . . is antiper*University Chemical Laboratory.
’Institut fur Anorganische Chemie. 0002-7863/84/ 1506-6200$01 S O / O
iplanar to the bond being broken either in the ground state or in some reasonably readily accessible higher energy conformation. (1) Chandrasekhar, S.; Kirby, A. J.; Martin, R. J. J . Chem. SOC., Perkin Trans. 2 1983. 1617. (2) Kirby, A. J.; Martin, R. J. J. Chem. S O ~ .Chem. , Commun. 1978, 803; 1979, 1079. (3) Kirby, A. J.; Martin, R. J. J . Chem. SOC.,Perkin Trans. 2 1983, 1627.
0 1984 American Chemical Society
J . Am. Chem. SOC.,Vol. 106, No. 21, 1984 6201
Bond Length and Reactivity in Acetals and Glucosides Table I. Geometry a t the Acetal Center of Axial Tetrahydropyranyl Acetals
bond lengths, compound
n
a
5a, Ar = Ph 3, Ar = 4-chlorophenyl 6a, X = H 3, Ar = 2,5-dinitrophenyl 6a, X = N O 2 3, Ar = 3,5-dinitrobenzoyl
1.448 (3)' 1.405 (3) 1.430 (3) 1.398 (3) 1.436 (5) 1.385 (4) 1.383 (5) 1.445 (5) 1.377 (4) 1.433 (5) 1.414 (6) 1.379 (7) "Numbers in parentheses represent standard deviations in
A
bond angles
d
b Y 1.433 (3) 1.378 (3) 114.0 111.6 118.9 1.427 (3) 1.369 (3) 113.7 111.7 119.6 1.364 (4) 114.1 105.9 1.448 (4) 119.6 1.354 (5) 113.8 110.1 118.8 1.458 (5) 1.351 (4) 114.9 105.4 118.6 1.466 (4) 1.476 (7) 1.328 (4) 113 8 106.6 117.1 the final figure quoted. Structural parameters are defined X
a
dihedral angles
D2
Dl
ref
64.6 66.9 11 55.1 64.3 16 67.5 158.1 14 66.2 55.1 26 69.2 164.0 15 -69.9 -110.8 29 in the text and in formula 9.
Table 11. Geometrv at the Acetal Center of Eauatorial Tetrahvdrouvranvl Acetals ~~~~~
bond lengths bond angles n X d ff R Y 111.5 107.3 114.1 1.392 (4) 1.417 (4) 2, R = N-phthalimidomethyl 1.418' (4) 1.419 (4) 5e, Ar = Ph 1.437 (3) 1.411 (3) 1.415 (3) 1.386 (3) 111.4 107.0 117.4 5e, Ar = 4-nitrophenyl 1.448 (4) 1.412 (4) 1.424 (4) 1.374 (4) 110.3 107.2 118.2 7e, R = 2,4-dinitrophenyl 1.449 (4) 1.411 (5) 1.448 (5) 1.350 (5) 110.6 123.2 105.7 7e, R = 3,5-dinitrobenzoyl 1.447 (3) 1.416 (3) 1.468 (3) 1.330 (3) 111.3 121.1 98.0 1.565 (3) 110.6 127.2 1.457 (2) 7e, R = P(O)(OPh), 1.465 (3) 1.408 (3) 105.2 104.9 127.0 1.461 (3) 1.413 (3) 1.456 (2) 1.567 (3) 110.5 125.9 7e. R = S 0 , M e 1.458 (31 1.403 (31 1.478 (31 1.568 (2) 110.6 106.7 "Numbers in parentheses represent standard deviations in the final figure quoted. Structural parameters are defined comvound
a
dihedral angles
D1 178.7 --177.4 176.8 175.9 --179.4 178.2 178.4 176.1 in the text
D, ref 71.4 30 76.8 12 -66.9 13 68.8 17 176.0 27 -44.7 31 -49.2 34.4 32 and in formula 9.
Table 111. Geometrv a t the Acetal Center of a-Glucouvranosides"
bond lengths
bond angles
dihedral angles D, D2
a n X d ff B Y ref 1.434 (4) 1.414 (4) 1.411 (4) 1.430 (4) 114.0 112.6 113.0 b 1.428 (2) 1.414 (2) 1.401 (2) 1.422 (2) 113.5 113.0 113.9 59 63.0 c 8a, X = Ac, R = Me 1.435 (5) 1.427 (7) 1.371 (9) 1.458 (8) 112.9 113.0 112.8 54.3 62.1 d 8a, X = H, R = Ph 1.436 (4) 1.407 ( 5 ) 1.419 (5) 1.386 (4) 115.0 112.5 86.2 18 117.0 -67.3 8a, X = H, R = 4-nitrophenyl 1.447 (3) 1.415 (3) 1.415 (3) 1.371 (3) 114.4 111.7 119.6 57.6 -61.2 20 1.440 (4) 1.414 (4) 1.420 (3) 1.372 (3) 112.8 111.3 117.7 85.6 -71.3 8a, X = R = Ac 1.422 (4) 1.403 (4) 1.431 (4) 1.354 (4) 114.6 110.2 118.2 -54.6 80.4 24 8a, X = Ac, R = 2,4-dinitrophenyl 1.429 (7) 1.411 (7) 1.417 (7) 1.352 (7) 114.8 110.0 120.8 -61.6 -69.8 33 'Structural parameters are defined in the text and in formula 9. bBerman, H. M.; Kim, S. H. Acta Crystallogr., Sect. B 1968, 824, 897. cNeutron diffraction data: Jeffrey, G . A.; McMullan, R. K.; Takagi, S. Acta Crystallogr., Sect. B 1977, B33, 728. d N o data are available for tetra-0-acetyl methyl a-glucopyranoside. These are data for the corresponding 6-methylsulfinyl compound (methyl tri-0-acetyl-6-deoxy-6-methylsulfinyl (S)-a-D-glucopyranoside of : Lindberg, K. B. Ibid. 1976, B32, 2017.
compound
8a, X = H, R = Me
Thus, axial tetrahydropyranyl acetals 1, which have such a lone pair on the ring oxygen, are readily hydrolyzed with loss of the OR group, whereas the equatorial isomers (2) are very stable if-but only if-their conformation is fixed. OR
EToR I
2
We chose to work with aryl tetrahydropyranyl acetals 3 because they are electronically unsymmetrical. The sp2-hybridized oxygen atom of the OAr group is effectively more electronegative than the oxygen atom of the ring and thus a poorer T donor and a better u acceptor. Hence, cleavage of the ring ( 0 ) C - 0 bond is sup-
3
pressed and that of C-OAr is favored, to such an extent that compounds with electron-withdrawing substituents in the Ar group undergo spontaneous cleavage in aqueous solution.* As part of this work we measured crystal structures of a small number of axial and equatorial tetrahydropyranyl acetals and observed that the C-OAr bond lengths in axial compounds (3) are remarkably sensitive to the nature of the OAr group.' This effect appeared to be absent not only in the equatorial series (2) but also in aryl glycosides,I0 which are much less reactive than 2-(aryloxy)tetrahydropyrans. Our preliminary conclusion, therefore, was that this was an effect on ground-state structure related to reactivity and subject to similar stereoelectronic factors.' Moreover, our initial small set of data showed a simple correlation between reactivity in the hydrolysis reaction (for which the rate determining step is C-OAr cleavage, 3 4) and the length of the bond being cleaved.' We report here a more extensive investigation of the pattern of bond lengths at the acetal center for a much larger set of axial and equatorial aryl tetrahydropyranyl acetals. We have also measured crystal structures for the corresponding series of a- and @-aryl glucosides, since published structures for aryl glucosides referred to derivatives of a miscellaneous selection of sugars.'O
-
4
(4) Kirby, A. J.; Martin R. J.; J. Chem. SOC.,Perkin Trans. 2 1983, 1633. (5) Briggs, A. J.; Evans, C. M.; Glenn, R.; Kirby, A. J. J . Chem. SOC., Perkins Trans. 2 1983, 1637. (6) Kirby, A. J. "The Anomeric Effect and Related Stereoelectronic Effects at Oxygen"; Springer-Verlag: West Berlin, Heidelberg, New York, 1983. (7) Deslongchamps, P. Heterocycles 1974, 7 , 1271. Tetrahedron 1975, 31, 2461. Pure Appl. Chem. 1975, 43, 351.
(8) Craze, G. A.; Kirby, A. J. J . Chem. SOC.,Perkin Trans. 2 1978, 354. (9) Jones, P. G.; Kirby, A. J. J . Chem. SOC.Chem. Commun. 1979, 288. (10) Ueno, K.; Saito, N.;Sato, M. Bull. Chem. SOC.Jpn. 1978, 51, 3170. Makinen, M. W.; Isaacs, N. W. Acta Crystallogr., Sect. 5 1978, 834, 1584. Harata, K. Ibid. 1976,832, 1932. Brehm, L.; Moult, T. Proc. R . Soc. London, Ser. E 1975,188,425. Tanaka, I.; Tanaka, N.; Ashida, T.; Kakudo, M. Acta Crystallogr., Sect. 5 1976, 832, 1559 Boles, M. D.; Taylor, D.J. Ibid. 1975, 531, 1400.
6202 J. A m . Chem. Soc., Vol. 106, No. 21, 1984
Briggs et ai.
Table IV. Geometry at the Acetal Center of 6-Glucopyranosides bond lengths compound a n X 8e, X = H, R = Me 1.440 (2) 1.433 (2) 1.379 (2) 8e, X = Ac, R = M e 1.427 (5) 1.431 (5) 1.385 (5) 8e, X = H , R = Ph 1.437 (6) 1.434 (5) 1.394 (6) 8e, X = H , R = 2-nitrophenyl 1.436 (4) 1.411 (3) 1.409 (4) 8e, X = H, R = 3,4-dinitrophenyl 1.435 (5) 1.405 (5) 1.405 (4) 8e, X = Ac, R = 3,4-dinitrophenyl 8e, X = R = Ac
1.431 (4) 1.435 (6) 1.421 (8)
1.397 (5) 1.395 (6) 1.426 (7)
1.410 (4) 1.400 (6) 1.408 (8)
d
a
bond angles 0
Y
dihedral angles
D,
D,
ref
1.430 (2) 1.443 (5) 1.388 (5) 1.368 (3) 1.364 (4) 1.355 (4) 1.375 (6) 1.344 (9)
111.5 112.6 111.8 111.3 111.3 112.2 110.3 111.9
108.1 107.3 107.4 106.9 107.4 107.4 109.1 105.8
113.1 112.6 118.0 118.8 119.3 117.8 117.8 117.5
173.9 173.9 179.7 -177.1 -179.9 -179.1 -172.9 177.6
-73.2 -76.7 82.5 -77.3 -78.1 -87.3 -82.3 -92.1
b c
19 21 22 23 25
“Structural parameters are defined in the text and in formula 9. bNeutron diffraction data: Jeffrey, G. A.; Takagi, S . A c t a Crystallogr., Sect. B 1977, B33, 738.
